1. Field of the Invention
The present invention generally relates to the detection of a zero crossing of a variable voltage and, more specifically, to the detection of the zero crossings of an A.C. voltage of known frequency.
The present invention, for example, applies to systems for detecting zero crossing of the mains voltage for controlling the operation of power converters, and especially of converters using (for example, as a controllable rectifying element) one or several power switches of thyristor, IGBT, etc. type.
The present invention more specifically applies to the case where the triggering of the power converter is desired to be controlled in the vicinity of a zero crossing to limit the surge current, without using a resistor having a high value.
2. Discussion of the Related Art
A recurrent problem of systems for detecting the zero crossing of an A.C. voltage provided by the mains is to avoid wrong detections due to microfailures making the mains voltage transiently disappear, or to bounces linked to the use of a mechanical switch on circuit powering-on.
FIG. 1 is a schematic block diagram of an exemplary power converter connected to the electric distribution network, of the type to which the present invention applies. For example, it may be a converter of switched-mode power supply type. The power converter is symbolized by a block 1 (PWC) in charge of supplying a load (not shown) with a voltage Vout. Generally, such a converter regulates voltage Vout on a predetermined reference value.
Converter 1 receives a D.C. voltage sampled across a filtering capacitor Cf connecting rectified output terminals 2 and 3 of a controllable bridge 4 having its A.C. inputs 5 and 6 receiving mains supply voltage Vac. In practice, a switch 7, generally a mechanical switch, is interposed between a first terminal 8 of application of voltage Vac and a first terminal 5 of bridge 4. Second A.C. input 6 of bridge 4 is connected to the second terminal 9 of application of voltage Vac. A mains filter (not shown) is further interposed between terminals 8 and 9 of application of voltage Vac and bridge 4.
In the example of FIG. 1, bridge 4 is a controllable bridge comprised of two thyristors Th1 and Th2 and two diodes D1 and D2. For example, thyristor Th1 is in series with diode D1 between terminals 3 and 2, the interconnection point corresponding to A.C. input terminal 5. Thyristor Th2 is in series with diode D2 between terminals 3 and 2, with terminal 6 as an interconnection point. Thyristors Th1 and Th2 are, in this example, cathode-gate thyristors and are controlled by a circuit 10 detecting the zero crossings (ZVS) of the voltage thereacross.
Other configurations are possible. In particular, the respective positions of the thyristors and of the diodes may be inverted. Similarly, additional switches, controlled according to the zero crossings of the A.C. voltage may be interposed at other circuit locations.
The function of circuit 10 for detecting the zero crossings is to turn on thyristors Th1 and Th2 each, at least at the starting, in the vicinity of the zero crossing of A.C. voltage Vac to avoid a turning-on of controlled bridge 4 in the middle of a halfwave, that is, under a high voltage. More generally, such a detection of the zero voltage relates to the zero crossings of the voltage between terminals of circuit 10, in practice across different elements of the converter. In the case of FIG. 1, this detection is performed across the actual power switches, circuit 10 providing control signals to the gates of thyristors Th1 and Th2.
FIG. 2 shows a conventional example of a circuit 10 for detecting the zero crossings of an A.C. voltage of the type illustrated in FIG. 1. In FIG. 2, the thyristors controlled by circuit 10 have also been shown, but for the fact that they are here assumed to be in the high stage of the rectifying bridge, that is, at the respective locations of diodes D1 and D2, which illustrates an alternative assembly with respect to FIG. 1.
In the representation of FIG. 2, thyristors Th1 and Th2 have their respective anodes connected to terminals 5 and 6 and their cathodes connected to terminal 2. The low portion of the bridge, formed, for example, of diodes, has not been illustrated. The assembly comprises a detection element for detecting the voltage difference between terminals 5 and 6 and a control element for controlling switches Th1 and Th2.
The respective gates of thyristors Th1 and Th2 are connected to the junction point of a MOS power transistor M and of a resistor R2. The gate of transistor M is connected to terminal 2 by a capacitor C and to the anode of an auxiliary thyristor Th3, triggered by the element for detecting the zero crossing of the voltage at terminals 5 and 6. This detection element comprises two diodes D3 and D4 having their respective anodes connected to terminals 5 and 6 and having their cathodes interconnected at a node A of the assembly. A dividing bridge, formed of resistors R3 and R4 in series, connects points A and terminal 2. The junction point is connected to the gate of thyristor Th3. In practice, a zener diode DZ2 is interposed between this gate and resistor R3 to set a control threshold. Capacitor C (anode of thyristor Th3) is connected to terminal A by a resistor R1 and a diode D7 connects this terminal A to the source of transistor M having its drain connected to resistor R2. Transistor M ensures an impedance matching between capacitor C and resistor R2 and a control of the current in resistor R2 according to the voltage across capacitor C. A diode D is connected in parallel on capacitor C.
The values of resistors R1, R2, R3, and R4 are selected so that the voltage across capacitor C is greater than the voltage across resistor R3, independently from the voltage difference between terminals 2 and A.
The function of capacitor C is to damp the abrupt variations of the supply voltage to avoid, due to thyristor Th3, the turning-on of one of thyristors Th1 or Th2. Thyristor Th3, when on, prevents the triggering of a thyristor Th1 or Th2 since it discharges capacitor C, preventing the turning-on of transistor M.
A circuit for limiting the surge current and controlling power switches of a rectifying bridge such as illustrated in FIG. 2 is described in U.S. Pat. No. 6,222,749, which is incorporated herein by reference.
A disadvantage of this solution is that it is difficult to integrate due to the large number of analog components used.
Another disadvantage is the use of a high-voltage MOS transistor (M).